Conventionally, a heat roller fixing method has been generally used in a fixing apparatus incorporated into an image forming apparatus using an electrophotographic process. A copying machine and a printer are examples of such image forming apparatus. In the heat roller fixing method, two fixing rollers, at least one of which is heated, are press-contacted to each other. A paper sheet (recording material) holding an unfixed toner image thereon passes between the two fixing rollers so that the toner image is fused and fixed on the paper sheet.
A method that has been adopted in recent years is a method in which a plurality of heaters (heat sources, heat members) are disposed inside the fixing roller to partially heat the fixing rollers. This method is adopted for the purposes of (i) reducing energy consumption of the image forming apparatus and (ii) improving life characteristics of the fixing rollers in the fixing apparatus.
One example of an arrangement in which a plurality of heaters are disposed is an arrangement in which two heaters, a main heater and a sub-heater, are disposed. The main heater heats a center section of a fixing roller, and the sub-heater heats both end sections of the fixing roller. The main heater heats a region of the fixing roller where a small-sized paper sheet passes among paper sheets processable in an image forming apparatus. The sub-heater heats end sections of the fixing roller through which a large-sized paper sheet passes.
In a surface temperature control of a fixing roller, the fixing roller is controlled so that a surface of the fixing roller keeps a preset fixing temperature. Generally, when a detected surface temperature of the fixing roller exceeds the preset fixing temperature, the heater is turned off. On the other hand, when the detected surface temperature is below the preset fixing temperature, the heater is turned on. In an arrangement in which a plurality of heaters are disposed, temperature sensors for detecting surface temperatures of the fixing roller are provided respectively in heating regions of the respective heaters. On the basis of the temperatures detected by the temperature sensors, power to the heaters corresponding to the respective temperature sensors are on/off controlled.
Moreover, conventionally, there have been suggested methods for preventing the occurrence of overshoot that is a phenomenon in which the surface temperature of a fixing roller temporarily takes a sudden rise right after the fixing roller is stopped. For example, Japanese Unexamined Patent Publication No. 95420/1996 (Tokukaihei 8-95420 (published on Apr. 12, 1996)) discloses an arrangement in which a heater stops heating a fixing roller at the same time as the fixing roller stops rotating, and the temperature starts to be controlled again after the surface temperature of the fixing roller starts to decrease.
However, in the arrangement in which a plurality of heaters are disposed inside the fixing roller, the surface temperature may exceed a destructive temperature at the end sections of the fixing roller. In such a case, the surface layer of the fixing roller may be deteriorated or damaged due to heat.
In the case of heaters whose heat generating regions that give off heat are different from each other, including the main heater and the sub-heater, positions that are in no need of heating in a fixing roller are unnecessarily heated.
FIG. 7 illustrates a relationship between a position of the fixing roller in its axial direction and a heat supply rate in the main heater and the sub-heater. On the assumption that heat supplied by the main heater is 100% at the center section of the fixing roller where the main heater needs to heat, substantially 25% of heat is supplied to the end sections of the fixing roller where the main heater does not need to heat. Similarly on the assumption that heat supplied by the sub heater is 100% at the end sections of the fixing roller where the sub-heater needs to heat, substantially 20% of heat is supplied to the center section of the fixing roller where the sub-heater does not need to heat.
Due to such an unnecessary heat supply to the fixing roller, the surface layer of the fixing roller may be destroyed when continuous printing is carried out on small-sized paper sheets.
FIG. 8(a) illustrates a relationship between a position of the fixing roller in its axial direction and a surface temperature of the fixing roller right after or during the continuous printing using small-sized paper sheets. As illustrated in FIG. 8, the surface temperature of the fixing roller decreases because heat of the fixing roller is taken away by the paper sheet passing between the fixing rollers. Thus, the surface temperature of the center section of the fixing roller becomes lower than the preset fixing temperature of the center section. The decrease in temperature at the center section of the fixing roller is detected by a center section temperature sensor for detecting a temperature at the center section of the fixing roller. As a result, the main heater is turned on.
On the other hand, decrease in surface temperature does not occur at the end sections of the fixing roller where the paper sheet does not pass through. Accordingly, the sub-heater is kept off in accordance with a temperature detected by an end section temperature sensor for detecting a temperature at the end section of the fixing roller. In theory, the end sections of the fixing roller are kept at the preset fixing temperature until the temperatures at the end sections of the fixing roller decrease due to heat dissipation.
However, in practice, as illustrated in FIG. 8(a), the surface temperatures at the end sections severely exceed the preset fixing temperature. This is due to unnecessary heat supply from the aforesaid main heater. Specifically, because the paper sheets continuously pass the center section, the main heater is kept on, during which the main heater supplies unnecessary heat to the end sections of the fixing roller. Accordingly, the surface temperature of the fixing roller gradually rises at the end sections although the sub-heater is turned off.
The range of temperatures that exceed the preset fixing temperature increases with increase in printed paper sheet count. If the printed paper sheet count increases further, the surface temperature of the fixing roller exceeds a destructive temperature at the end sections of the fixing roller, as illustrated in FIG. 8(b). This destroys the surface layer of the fixing roller.
The destruction of the surface layer of the fixing roller can be prevented before it happens, with an arrangement in which: (i) a destruction prevention temperature is set at a temperature a little lower than the destructive temperature and (ii) the operation of the image forming apparatus is stopped emergently at a point in time when the end section temperature sensor detects that surface temperature of the end section of the fixing roller has reached a destruction prevention temperature that is preset to be a little lower than the destructive temperature. However, such an arrangement leads to decrease in operation rate of the image forming apparatus.
The aforesaid Japanese Unexamined Patent Publication does not consider the arrangement in which a plurality of heaters are disposed inside the fixing roller. Accordingly, the aforesaid Japanese Unexamined Patent Publication cannot solve the problem mentioned above.